Harden native Auto Gain filter

[Avejack]

Fixes #46

Summary

• replace the capture Auto Gain implementation with a stateful AGC using frame-count/sample-rate based smoothing
• add noise-floor gating, headroom limiting, PCM-safe sample writes, and read-only AGC telemetry
• expose the new Auto Gain parameters and metrics through the Dart filter API
• add Dart metadata tests and an offline C++ fixture for callback-size invariance, silence handling, limiting, s16 full-scale safety, and read-only metrics
• regenerate web WASM artifacts and update the Android example build tooling in separate commits

Testing

• dart analyze
• flutter test --no-pub test
• g++ -std=c++17 -Wall -Wextra -Wpedantic -I src test/cpp/autogain_test.cpp src/filters/autogain.cpp -o /tmp/flutter_recorder_autogain_test && /tmp/flutter_recorder_autogain_test
• cmake --build /tmp/flutter_recorder_cmake_build
• cd web && sh ./compile_wasm.sh using emsdk at /home/Sean/projects/emsdk
• git diff --check

Notes

• The first six Auto Gain parameter indices remain in the same order; the new parameters and metrics are additive.
• Runtime metrics are read-only through the existing filter parameter lookup path.

[alnitak]

This is gorgeous PR @Avejack!

Thank you very much!

LGTM, merging and updating on pub.dev.